1. Field of the Invention
The present invention is related to trimming a programmable resistor to a predetermined resistance, and especially to trimming an electronic fuse (e-fuse) to a predetermined resistance.
2. Description of the Prior Art
An e-fuse is an electronic fuse (poly-fuse) that can be broken by an electrical current. Traditional e-fuses are used in redundancy circuits of semiconductor memory devices. The redundancy circuit includes a plurality of e-fuses. When a defective memory cell is found and needs to be avoided, the corresponding poly-fuse is broken by an electrical current, and the defective memory cell is replaced by a redundant memory cell to allow the memory to function properly.
Please refer to FIG. 1. FIG. 1 illustrates the device structure of such an e-fuse. The traditional e-fuse typically includes a poly-fuse strip 12 with one end serially connected to a source/drain 18 of a MOS transistor 16, and the other end connected to a positive voltage (i.e. the fuse source voltage, VFS). The MOS transistor 16 has another source/drain 20 that is connected to ground, and a gate 22 that is biased to a gate voltage (Vg) to allow an electric current to flow through the poly-fuse 12 and blow the poly-fuse 12 in a very short time. Generally, the gate includes a polysilicon layer, a silicide layer, and a silicon nitride cap layer. The poly-fuse 12 has structure similar to the gate, that is, a stack of a polysilicon layer, a silicide layer, and a silicon nitride cap layer.
Referring to FIG. 2, a gate voltage (Vg) vs. time plot showing a pulse voltage waveform applied to a gate of a MOS transistor of an e-fuse device to be blown according to prior art method is illustrated. As shown in FIG. 2, the pulse voltage waveform is a single-level square waveform that can be generated by a conventional voltage pulse generator. The simple pulse voltage waveform in FIG. 2 has a maximum voltage value VIH during time period T2-T1, and a minimum voltage value VIL that is typically 0 volts. Ordinarily, the maximum voltage value VIH is about the threshold voltage of the MOS transistor. When the e-fuse is blown, the cobalt and boron inside migrate out. At this time, the resistance of the e-fuse will rise. By controlling the time period T2-T1, the cobalt and boron can be moved out thoroughly so part of polysilicon is depleted. As a result, the resistance of the poly-fuse becomes very high, and even becomes an ultra-high resistance. For example, a poly-fuse strip of an e-fuse device having a resistance of about 100 ohms before being blown may have a resistance of about one megohm after being blown.
In the conventional method, the e-fuse generally has two states, the unblown state in which the poly-fuse strip has low resistance; and the blown state, in which the poly-fuse strip has an ultra-high resistance. Due to this characteristic, e-fuses have also been applied in programmable read-only memories (PROMs) to facilitate input of data. When a “1” is transmitted into the PROM, a high voltage is applied to the gate to blow the poly-fuse into a megohm resistance to form an off-state. Otherwise, the poly-fuse is not blown, and the circuit is at an on-state when a “0” is transmitted into the PROM. The fuse blowing process using input voltages is programming. Through the programming process, the e-fuse is able to save data as a digital bit. However, once the poly-fuse is blown, it becomes an open circuit (off-state) forever. In another words, the e-fuse is unrecoverable.